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Niwa scientists on a research expedition in the South Taranaki Bight spotted nearly 50 blue whales in the past week.

Sightings of the world's largest animal are reasonably rare.

The World Wildlife Foundation estimates the worldwide population to be about 10,000 to 25,000.

That was down from more than 200,000 in the early 20th century.

Niwa marine ecologist Leigh Torres led a team of blue whale researchers in the bight on a voyage aimed at collecting critical data to enhance understanding of the blue whale population in the region.

"It is very exciting to see these whales and start the process of collecting important data on this undescribed population and poorly understood foraging habitat," she said.

"In addition to finding the whales, we were able to detect their prey visually on the surface and at depth using hydro-acoustics."

It was previously thought the whales travelled through New Zealand waters only while migrating.

However, research by Torres suggests they were there to feed on plankton, which the researchers were now in the process of documenting.

"Blue whales need to eat vast amounts of plankton to support their energy demands.

"But there are just four confirmed blue whale foraging grounds in the southern hemisphere outside of Antarctic waters," Torres said.

Blue whale scientist Pete Gill said the whales exhibited similar behaviour patterns to the blue whales seen feeding around Australia.

The research team has taken hundreds of photos to identify the individual whales, tissue samples to genetically identify the individual whales and their prey species, acoustic recordings, prey samples and oceanographic data.